Stresses such as ischemia or ATP depletion induce the synthesis of cytoprotective heat stress proteins (HSP). Of the inducible cytoprotectants, HSP 72 is the most abundant and well-characterized. HSP 72 is a molecular chaperone that removes and repairs damaged cell proteins. In non-renal cells and tissues, selective over-expression of HSP 72 decreases subsequent injury from ischemia or ATP depletion. In these models, the cell functions protected by HSP 72 have not been identified. In the renal epithelial cell, ischemia in vivo or ATP depletion in vitro induce HSPs, cause collapse of the actin cytoskeleton, loss of tight junction integrity, impair mitochondrial ATP production and trigger apoptosis. We have previously shown that prior heat stress, sufficient to upregulate HSP 72, preserves these cell functions and improves renal cell survival after ATP depletion. The central hypothesis of this project is that HSP 72 is a major renal cytoprotectant. We suggest that by binding to proteins that regulate: (a) the cytoskeleton, (b) cell-cell contact sites, (c) native DNA, (d) mitochondrial function and (e) the apoptotic pathway, HSP 72 decreases cell injury and inhibits apoptosis caused by ATP depletion. This hypothesis is supported by our preliminary studies in which the selective induction (or suppression) of HSP 72 alone alters survival in ATP depleted renal epithelial cells. In an in vitro model of ATP depletion that resembles ischemia/reperfusion in vivo, we will: (1) identify cell functions protected by HSP 72; (2) identify some of the intracellular proteins that interact with HSP 72; (3) determine the role of protein binding vs. refolding in mediating cytoprotection by HSP 72 and (4) evaluate intracellular signal(s) that can induce HSP 72 without requiring heat stress. We have developed the molecular, immunologic and biochemical tools to manipulate HSP 72 content in renal epithelial cells and to identify proteins that bind to it. Human HSP 72 deletion mutants with well-described defects in protein binding or re-folding will be used to determine the mechanism by which this molecular chaperone protects target proteins in ATP depleted cells. To evaluate some of the intracellular signals that induce HSP 72, we have developed a novel, permeabilized cell system in which intracellular pH, calcium and adenine nucleotide content can be individually manipulated. These studies will examine some of the pathways that permit HSP 72 to protect the renal epithelial cell and may ultimately provide a means for increasing cellular resistance to ischemic injury, a common cause of acute renal failure.